Method and device for detecting phase failures, in particular network faults, in a converter

ABSTRACT

In a method and to a corresponding device for detecting phase failures in a converter, current regulators of a positive phase sequence system and current regulators of a negative phase sequence system are provided for the current control of the converter, wherein the current regulators of the positive phase sequence system and the current regulators of the negative phase sequence system each have an integrator, resulting, in the case of a network fault, in coupling of the integrators. At least one measured or calculated value is checked by a monitoring unit for a course that is typical of the coupling of the integrators, wherein the monitoring unit generates a fault signal if such a typical course is detected.

The invention relates to a method and device for detecting phasefailures, in particular phase failures in the form of line failures, ina converter, in particular in a converter connected to a supplying line.

To protect expensive workpieces during the production process in amachine tool, phase failures, in particular line phase failures, must bedetected quickly and reliably and the equipment placed e.g. in a safeoperating state on the basis of such detection. This is in order toprotect the workpiece currently being processed e.g. in a multi-axislathe/milling machine from damage caused by unwanted/uncoordinatedmovement of the machine axes due to a power supply dip. Because thevalue of machined workpieces is sometimes in the six-figure or even inthe seven-figure range, a high outlay for protecting such workpieces isjustified.

The approach described in the following is suitable for use in machinetools, particularly metal-cutting machine tools, and other productionmachines. The term machine tool commonly refers to all machines whichare used among other things in mechanical engineering and toolmaking forprocessing workpieces using tools. On the other hand, an industrialrobot is an all-purpose, programmable production machine which isdesigned and set up not only to process workpieces but alternativelyalso to handle workpieces and for assembly purposes. Here and in thefollowing, the expression machine tool is used generically for termssuch as machine tool, production machine, industrial robot and the like.

If an impending line failure or disturbance is suspected, precautionaryretracting movements of the machine axes of a machine tool shall beinitiated. The overall concept of the machine tool must be geared tothis requirement. Machine tool manufacturers demand correspondingfunctions from the integrated drive and control units.

In such applications, a self-commutated IGBT bridge converter istypically used as a dynamic line inverter with feedback capability. Theconverter controls the DC link voltage of the drive group and impresses(e.g. using pulse width modulation PWM) sinusoidal line currents whichresult in the necessary power flow to the line. In order to keep thepulse frequency components of the converter output voltage away fromother line users, line filters are used which typically containinductors and at least one capacitor in the filter branch. The converteris typically controlled such that the reactive power demand at theconnection point of the filter is virtually zero, i.e. apart from thehigher-frequency distortion components, pure active current is exchangedwith the line (displacement factor cos φ=1). The reactive power demandof the capacitor in the line filter is provided for this purpose by theconverter.

The comparatively frequently occurring event of a line voltage dip or ofa short circuit or ground fault does not need to be considered in thefollowing, as the associated large voltage difference between inverterand dipped supply voltage results in a large current which can be easilydetected. In addition, large variations in the voltage amplitude can beeasily detected using voltage sensing and/or model calculations of theconnection voltage.

On the other hand, a different situation arises in the event of ahigh-resistance failure of a line phase. A typical example of this isthe tripping of a cutout in a line phase. The voltage of the lost linephase is retained and simulated by the capacitor of the line filterrequired for operating the converter. Particularly under no-loadconditions, the detection of a high-resistance line failure thereforebecomes a problem for which a solution is proposed here. On the basis ofthe thus possible detection of a high-resistance supply failure, amachining step can be preventing from starting in the machine tool, forexample, if a branch cutout has previously tripped.

The high-resistance failure of more than one line phase is—because ofinterrupted energy flow of the 3-conductor connection—once againinsignificant and will not therefore be considered here. For example,such a situation could be detected from the interrupted current flow orfrom the abruptly changing voltage at the line filter.

Although the approach presented here is explained taking the example ofa line-side use of a PWM inverter, it is equally applicable tomotor-side inverters with sine wave output filter (having longitudinalinductance and transverse capacitance). The topology and the controlmethods can be used in a basically similar manner, merely replacing theblock “line” by an electrical motor, in particular a synchronous motor.This can be explained by the fact that line supplies are typicallycreated using electrical generators, which means that, even in the caseof a line inverter, the block “line” can consist of an electricalmachine.

An object of the present invention is to specify a method for detectingphase failures, in particular phase failures in the form of single-phaseline phase interruptions hereinafter referred to a line faults forshort, in particular for detecting line faults during no-load orpart-load operation of active line inverters with line filters.

This object is achieved according to the invention by means of a methodhaving the features as claimed in claim 1 and by means of a devicehaving the features as claimed in the parallel device claim.

For the method for detecting phase failures in a converter, particularlyline faults, namely single-phase line phase interruptions, in aconverter connected to a supplying line, on the one hand currentcontrollers of a positive phase sequence system and, on the other hand,current controllers of a negative phase-sequence system are provided forcurrent control of the converter, wherein both the current controllersof the positive phase sequence system and the current controllers of thenegative phase sequence system have integrators or integral-actioncomponents. The positive phase sequence system and the negative phasesequence system are designed such that the integrators of the respectivecurrent controllers are normally, i.e. in the absence of a line fault,decoupled. In the event of a line fault, coupling of the hithertodecoupled integrators and therefore a deliberate instability results.This instability produces, for different measured or calculated values,a response that is typical of the coupling of the integrators and henceof the underlying line fault. The response of the or each measured orcalculated value in question is checked by means of a monitoring unitand the monitoring unit detects a response of the respective measured orcalculated value that is typical of a line fault. An example of apossible measured or calculated value for detecting a line fault are theabsolute values of the integrators of the current controllers of thenegative phase sequence system. In the event of such a response typicalof a network fault being detected, the monitoring unit generates a faultsignal so that operating personnel are made aware of the fault situationand/or an automatic reaction to the line fault, e.g. In the form ofshutdown of the machine tool, becomes possible.

A device designed and set up to carry out the method, namely to detectphase failures in a converter, in particular line faults in a converterconnected to a supplying AC line, comprises, on the one hand, currentcontrollers of a positive phase sequence system and, on the other hand,current controllers of a negative phase sequence system for currentcontrol of the converter, wherein the current controllers of thepositive phase sequence system and the current controllers of thenegative phase sequence system have integrators or integral-actioncomponents. As already explained above, the positive phase sequencesystem and the negative phase sequence system are designed such that theintegrators of the respective current controllers are normally, i.e. inthe absence of a line fault, decoupled. However, in the event of a linefault, coupling of the integrators results and a response of one or moremeasured or calculated values that is typical of the coupling of theintegrators can be detected by means of a monitoring unit. In the eventof such a typical response being detected, the monitoring unit can alsogenerate a fault signal which can be evaluated, for example, forautomatic reactions to the fault situation detected.

The advantage of the invention is that phase or line faults of the typementioned in the introduction, which typically arise because of trippingof a line cutout, can be detected even before the start of a productioncycle and, on this basis, a timely warning or shutdown of a machine,e.g. a machine tool, connected to the line inverter is possible. In thecase of a machine tool connected to the line inverter, this provides aneffective means of protecting workpieces from damage which may otherwiseoccur due to aborted production processes.

Another advantage of using the approach described here is that normaloperation of the line inverter is not interrupted or disturbed by testsignals for fault detection, as is otherwise necessary to preventreactions on the line voltage. Idle times in the production process arenot therefore incurred. Moreover, the proposed method is basically alsoapplicable to phase failure detection for motor inverters with sine waveoutput filter.

Other advantages of the invention are a short detection time—a linefault can typically be detected in less than one second using theapproach described here—and negligible effects on the stability androbustness of the usual current and DC link voltage control. This is dueprimarily to the fact that no disturbing line effects arise due to theoutputting of test signals or similar that are often necessary foractive detection methods. The method is robust and reliable and allowsassured detection without erroneous activation even in the event ofsupply voltages overshoots or supply impedance changes. Lastly, themethod is all-purpose, i.e. it can be used both for “small” inverters inthe kW range and also for “large” inverters in the MW range with/withoutvoltage measurement in the line filter and different filter topologies.Also the low computational requirement of the method makes it usable inthe booksize environment in multi-axis groups for machine tools withoutlimitation of the maximum number of axes per control unit.

Advantageous embodiments of the invention are set forth in thesub-claims. Back-references used in said sub-claims relate to furtherrefinements of the subject matter of the main claim by virtue of thefeatures of the particular sub-claim. They are not to be understood as awaiver of the achievement of independent, objective protection for thecombination of features of the sub-claims to which they refer. Inaddition, having regard to an interpretation of the claims in the caseof a more detailed concretization of a feature in a subordinate claim,it is to be assumed that such a restriction does not exist in therespective preceding claims. Lastly it is pointed out that the methodspecified here can also be further developed according to the dependentdevice claims and vice versa.

In an embodiment of the method, limiting blocks are provided in thenegative phase sequence system to limit the ramp-up of the integratorsof the negative phase sequence system. By means of the limiting blocks,the output of the negative phase sequence system control circuit can belimited, thus allowing fault ride-through, in particular of the linefault, while still ensuring reliable detection.

In another embodiment of the method it is provided that the negativephase sequence system is not continuously active and is activated as andwhen required depending on an average power consumption of theconverter, e.g. only under no-load or partial load conditions. Thistakes account of the fact that if the line currents are sufficientlylarge, a line fault in the form of a phase separation can also bedetected in other ways, e.g. on the basis of the harmonic components attwice the line frequency in the DC link voltage, of the instantaneouspower, of the active current, of the reactive current, etc. Suchvariables can be easily evaluated in per se known manner using abandpass filter and threshold value monitoring of the bandpass filteroutput.

In yet another embodiment of the method, the negative phase sequencesystem is deactivated if the average power consumption of the converterexceeds a predefined or predefinable threshold value. Such adeactivation of the negative phase sequence system control can beuseful, for example, in order to increase the stability of the overallcontrol in the case of high power or to reduce the computational load.

The above stated object is also achieved using a control device actingas a converter control unit for controlling a converter, said controldevice operating according to the method as described here and in thefollowing and comprising, for this purpose, means for carrying out themethod. The invention is preferably implemented in software or insoftware and firmware. The invention is therefore on the one hand also acomputer program comprising program code instructions executable by acomputer and, on the other hand, a storage medium containing such acomputer program, i.e. a computer program product with program codemeans, and finally also a converter control unit into whose memory sucha computer program is or can be loaded as a means of carrying out themethod and the embodiments thereof. Where method steps or method stepsequences are described in the following, this relates to actions whichtake place automatically on the basis of such a computer program orautomatically under the control of the computer program.

Instead of a computer program having individual program codeinstructions, the method described here and in the following can also beimplemented in firmware form. It will be clear to persons skilled in theart that instead of an implementation of the method in software, animplementation in firmware or in firmware and software or In firmwareand hardware is always possible. For the description presented here,this means that the term software or the term computer program alsoincludes other possible implementations, namely in particular animplementation in firmware or in firmware and software or in firmwareand hardware.

An exemplary embodiment of the invention will now be explained ingreater detail with reference to the accompanying drawings. Mutuallycorresponding items or elements are provided with the same referencecharacters in all the figures.

The exemplary embodiment is not to be understood as a limitation of theinvention. This applies particularly with regard to the description onthe basis of a line fault. As mentioned in the introduction, theapproach can equally be used for motor-side converters. This is alwaysimplicit in the following. Moreover, additions and modifications arealso completely possible within the scope of the present disclosure, inparticular those which e.g. by combination or variation of individualfeatures or method steps described in connection with those in thegeneral or specific section of the description and contained in theclaims and/or accompanying drawings will be understood by personsskilled in the art in respect of achieving the stated object and,through combinable features, result in a new item or new method steps Imethod step sequences.

FIG. 1 shows a system comprising a converter,

FIG. 2 shows another illustration of the system according to FIG. 1 withadditional details,

FIG. 3 shows graphs of characteristic variables of the system accordingto FIG. 1, FIG. 2,

FIG. 4 shows a system according to FIG. 1, FIG. 2 comprising a convertercontrol unit and

FIG. 5 shows graphs of characteristic variables of the system accordingto FIG. 4.

FIG. 1 shows a simplified illustration of a system (converter group)comprising a line inverter 10 referred to in the following as aninverter, power converter or converter 10 for short, having a linefilter 12 with commutating reactor (longitudinal inductance) and filtershunt arm with a capacitor (transverse capacitance) and havingconnecting lines with disconnecting points 14 to a supplying line 16 towhich other components 18 may also be connected. It has already beenexplained in the introduction that the approach proposed here is alsoapplicable to motor-side inverters. In which case the entity referred toas the line 16 here would be understood to be a motor.

FIG. 2 shows a simplified representation of the system according to FIG.1 with additional details, namely the pulsed/switched, in particularpulse-width-modulated output voltage U_(U) of the converter 10 as wellas commutating reactor, line filter shunt arm with capacitor andpossibly other components 18 (here additionally with neutral grounding),connection point with disconnecting point 14, concentrated lineinductance and ideal line voltage source U_(N). A high-resistance supplyfailure is simulated by single-phase opening at the disconnecting point14 between line filter 12 and line inductor. Depending on the design ofthe line filter 12, instead of or in addition to the use of reactors inthe filter shunt arm, an inductance can also be present between shuntarm and disconnecting point.

The problem to be addressed by the invention can be simply illustratedby the circuit according to FIG. 2: As the converter 10 regulates to cosφ=1 at the connection point to the line 16 (disconnecting point 14),under no-load conditions (P_(Wirk)≈0) the converter current isapproximately equal to the filter current. Then i_(U,RST)≈i_(C,RST) andi_(N,RST)≈0. Obviously this limit case is not exactly attained, as atleast the intrinsic losses within the system must be covered from theline current i_(N,RST). However, it becomes clear that an open phase atno-load has only minimal effect on the current and voltage relationshipsat the converter 10.

The detection of a high-resistance failure of a line phase is designedto operate even without measurement of the filter voltages u_(F,RS), undu_(F,ST), as a device-external voltage measurement required for thatpurpose is not generally provided in small converters 10 for reasons ofcost. However, even if these measured variables were available, it wouldbe of little help: The capacitor voltage at a high-resistance linebranch will approximately follow the fundamental component response ofthe corresponding converter output voltage and will not therefore ensurereliable state detection. This is the case at least as long as theconverter 10 in pulsing mode is acting as a voltage source. On the otherhand, under pulse inhibition (the IGBT inverter then operates merely asa diode rectifier) the missing line voltage can be unambiguouslymeasured.

The larger the instantaneous apparent power of the converter group, thegreater the obviously resulting current and voltage deviations from anexpected response. This explains how, above a partial load threshold,phase failure detection is ensured using the monitoring facilitiesalready implemented today (deviation of the line phase angle from theexpected response, deviation of the active current and DC link voltagefrom the expected response).

To illustrate the underlying problem, FIG. 3 shows a measurement on areal test set-up, namely (from top to bottom) the phase currents betweenconverter 10 (active line module; ALM) and line filter 12 (section I),the integral-action components of active and reactive current controller(section II), the DC link voltage V_(DC) (section III), the measuredfilter voltages u_(F,RS) and u_(F,ST) (section IV), and the degree ofsaturation, i.e. the output voltage of the converter 10 referred to theDC link voltage (section V). At time t=96 ms, the phase S is opened, i.ethe variable i_(N,S)=0 is switched. As the reactive current requirementof the line filter 12 is impressed by the converter 10 and the otherline currents i_(N,R) and i_(N,T) are therefore also very small, therearise no significant changes in system variables which would be usablefor robust detection of a line fault. This also applies particularly tothe filter voltages u_(F,RS) and u_(F,ST) as well as the phase currentsi_(U,R), i_(U,S) and i_(U,T) of the converter 10. At least the latterare generally available as measured variables; in many applications thefilter or rather line voltages are only calculated using models.

The approach proposed here uses the method of segmenting a three-phasesystem into symmetrical components. This mathematical approach meansthat a three-phase voltage/current system with any, i.e. even unequalamplitudes, can be broken down into a plurality of voltage/currentsources which each have the same amplitude in all three branches (i.e.are symmetrical). The three-phase system is subdivided into azero-sequence system which acts identically in all three brancheswithout mutual phase displacement, and two three-phase voltage/currentsystems each having 120° phase displacement between the branches andpossessing a positive direction of rotation (positive phase sequencesystem 20) or a negative direction of rotation (negative phase sequencesystem 22).

FIG. 4 illustrates an embodiment of the converter group according toFIG. 1, FIG. 2, comprising a DC link voltage source 24 as well as apositive phase sequence system 20 and a negative phase sequence system22.

In the normal case of equal amplitudes in the three conductors and 120°phase displacement between the conductor variables, only the positivephase sequence system 20 is non-zero. This means that a current controlwhich impresses the currents in the positive phase sequence system 20(i.e. in the phase sequence predefined by the line 16) is adequate foruse in regular balanced AC lines.

Accordingly, the current controls of converters typically only possess acontrol loop for the positive phase sequence system 20 comprising ad-current controller 26 and a q-current controller 28. Using a networkanalyzer 30 or a PLL, the current line phase angle φ and the currentline voltage ν_(d,N), ν_(q,N), for precontrolling the converter outputvoltage are calculated from the measured currents and/or voltages. Bytransforming the three phase currents into □□ space vector coordinates,a clear representation in two axes is achieved (Clarke transformation).Another transformation by rotation with the line phase angle φ□ into d/qcoordinates (Park transformation) allows the use of steady-state controlwith integrator component, e.g. in the form of a PI controller or a PIDcontroller, to achieve steady-state accuracy.

Decoupling blocks 32, 34 compensate the coupling of the two space vectoraxes resulting from the vector rotation into the dq-system. The voltageoutput vector of the control is converted using a modulator 36 intoswitching commands for the semiconductor switches of the converter 10.In the time averaged over a switching period, the switching actions atthe three-phase converter output result in precisely the output voltagecalculated by the controller 26, 28. The line/sine wave filter 12(typically of LC or LCL design) smooths the fast switching-frequencyvoltage variations (typically in the kHz range) at the converter outputso that, at the point of common coupling to the line 16 (disconnectingpoint 14), approximately sinusoidal signal responses are produced whoseharmonic content is within the applicable guidelines and standards.

Of importance for the applicability of the method proposed here is thefact that the d-q current controllers 26, 28 have an integral-actioncomponent, i.e. a single pole at s=0 or very close to the origin of thecomplex s-frequency domain, (for digital controllers, the correspondingpoint is at z=1). It is known from systems theory that such transferelements are stable and do not increase beyond all limits as long as theinput variable, i.e. the control deviation, has no steady-state directcomponent. The d- and q-axes of the control constitute orthogonalcomponents, are therefore decoupled and can be controlled independentlyof one another. This explains why the controllers 26, 28 can eachcontain an integrator for both axes, but the control circuit is stable.

For the purpose of active phase failure detection, conventional currentcontrol is augmented by a control block for the negative phase sequencesystem 22 having separate current controllers 40, 42: The desired valuesfor the negative phase sequence system current are usually set to 0. Theactual current values of the negative phase sequence system currentform, as the control deviation, the input values for the two axes of thenegative phase sequence system controllers 40, 42. In the exemplaryembodiment, these actual values are efficiently calculated by rotatingthe current deviation Δi_(d), Δi_(q) from the positive phase sequencesystem 20 by double vector rotation with the phase angle φ□ (or bysingle vector rotation with twice the phase angle) into the d-qcoordinate system of the negative phase sequence system 22(corresponding to an oppositely oriented current/voltage system). Theadvantage of this is that the generally dominant harmonic component ofthe positive phase sequence system 20 has already been approximatelysubtracted, as the positive phase sequence system desired value has beenremoved. Alternatively, the actual current values of the positive andnegative phase sequence system 20, 22 could, for example, be extractedfrom i_(□) and i_(□) by bandpass filters in each case. Non-zero negativephase sequence system currents are represented in the positive phasesequence system 20 as a harmonic with twice the line frequency (and viceversa).

A critical factor for the operation of the method is that the d-qnegative phase sequence system current controllers 40, 42 likewise haveintegral-action components (i.e. poles at s=0 or z=l). Additionalproportionally or differentially acting controller elements may bepresent, but are not necessary. In this respect it is preferable for apurely integral-action controller to be provided in the negative phasesequence system 22, as the P(D) element of the positive phase sequencesystem controller 26, 28 is effective for the entire frequency range.Both the positive phase sequence system 20 and the negative phasesequence system 22 have an additional decoupling block 44, 46. The totaloutput voltage results from adding the output voltage of the negativephase sequence system controllers 40, 42 to the output voltage of thepositive phase sequence system controllers 26, 28.

In the fault-free balanced system, the converter currents in thepositive phase sequence system 20 and in the negative phase sequencesystem 22 constitute orthogonal components. These are decoupled and canbe controlled independently of one another. The integrators 50, 52; 54,56 of the controllers 26, 28; 40, 42 of the positive phase sequencesystem 20 and of the negative phase sequence system 22 are thereforelikewise decoupled and do not mutually produce resonance. On the otherhand, the disconnection of a line phase results in a structural changein the system, which causes the positive and the negative phase sequencesystem 20, 22 to be coupled. In the combined overall system of positiveand negative phase system currents, a plurality of poles are accordinglyproduced on the imaginary axis and therefore an instability arises whichresults in an easily detectable increase in the integral-actioncomponents. It can be shown that the integrators of the positive andnegative phase sequence system 20, 22 counteract one another in theevent of line phase separation, i.e. positive feedback is produced.

The integrators will preferably not be allowed to ramp up to the extentof producing a failure due to overcurrent or overvoltage. It istherefore provided to only allow the integral-action components to rampup until an alarm threshold for unambiguous detection is exceeded and anindication can be sent to a higher-order controller or operatingpersonnel. Using limiting blocks 60, 62 or anti-windup feedback loops tothe integrators 54, 56 of the d-q negative phase sequence system-currentcontrollers 40, 42, an unlimited ramp-up of the negative phase sequencesystem-integrator is prevented and stable operation maintained. Forexample, the detection threshold for the negative phase sequence system22 could be usefully set at 20% of the nominal connection voltage.Depending on the specific application, emergency retraction of the toolof the machine tool or even robust line fault ride-through can beimplemented if the detection threshold is reached or exceeded.

The approach proposed here is based on supplementing normal currentcontrol for a converter 10 having integrators for adjusting the real andimaginary part of the positive phase sequence system current withoutpermanent control deviation with a negative phase sequence system 22which itself comprises current controllers 40, 42 having integrators.The current controllers 40, 42 of the negative phase sequence system 22are designed to counterbalance the complex negative phase sequencesystem current. The integrators incorporated in the current controllers40, 42 ensure that the complex negative phase sequence system current iscounterbalanced without permanent control deviation. Converter controlof this kind is not normally necessary for symmetrical controlledsystems, as here no or only minimal excitations are present in thenegative phase sequence system 22 and currents and voltages in thenegative phase sequence system 22 remain small. Apart from exceptions,the desired value for the complex negative phase sequence system currentis set to 0. Even in the undisturbed symmetrical system, the negativephase sequence system control and particularly the integrators thereofhave a negligible effect on the system being controlled. In a disturbedsystem, i.e. a system having a disconnecting point 14 resulting from ahigh-resistance failure of a line phase, the integrators of the negativephase sequence system 22 act as an initially small, but continuouslyincreasing disturbance in the overall system, which allows the fault tobe easily detected.

To detect the fault, a monitoring unit 64 is provided. This is designedand set up to check significant measured und/or calculated variables fora response typical of a fault situation. A measured or calculated valueresponse typical of a fault is e.g. the exceeding of a predefined orpredefinable limit value. Measured or calculated values alternatively orcumulatively possible for monitoring by means of the monitoring unit 64are, for example

-   -   the absolute values of the integrators of the negative phase        sequence system control,    -   the oscillation amplitude of the line current absolute value at        twice the line frequency,    -   the oscillation amplitude of the filter voltage absolute value        at twice the line frequency,    -   the oscillation amplitude of the DC link voltage at twice the        line frequency or    -   the oscillation amplitude of the saturation degree at twice the        line frequency.

In the exemplary embodiment shown in FIG. 4, the absolute values of theintegrators of the negative phase sequence system control are monitoredby means of the monitoring unit 64. In the situation shown, the absolutevalues of the integrators are low-pass filtered by means of basicallyoptional PT1 elements connected upstream of the monitoring unit 64. Ifone of the absolute values or both absolute values exceed a predefinedor predefinable limit value, this is detected in basically per se knownmanner by means of the monitoring unit 64 and e.g. a fault signal 66 isgenerated by means of the monitoring unit 64.

On the basis of such a fault signal 66, a message such as “(line) phasefailure detected” can be generated for the operating personnel dependingon the requirements of the particular application. By means ofevaluation of the fault signal 66 by a higher-level controller (notshown), a suitable reaction can be automatically triggered in therespective process, e.g. a stop, a retraction movement, execution of anemergency program, forwarding of the information to higher-level controlstructures, etc.

An implementation of the functional units shown in FIG. 4, namely of atleast the positive phase sequence system 20 and the negative phasesequence system 22, acts as a converter control unit 68. The positivephase sequence system 20 and the negative phase sequence system 22 areimplemented e.g. in software, i.e. as a computer program. In thisrespect, the converter control unit 68 comprises a per se knownprocessing unit (not show here) in the form or manner of amicroprocessor, and a memory (likewise not shown). The memory is loadedwith the computer program containing the implementation of the positivephase sequence system 20 and of the negative phase sequence system 22and, during operation of the converter control unit 68, the computerprogram is executed by the processing unit, resulting in execution ofthe approach described here.

FIG. 5 lastly illustrate the same situation as shown previously in FIG.3, but using the detection system proposed here in the form of thenegative phase sequence system 22 with the associated controllers 40,42. Shown in a first top section is the DC link voltage VDC (section I).The next section down is the degree of saturation, i.e. the outputvoltage of the converter 10 referred to the DC link voltage (sectionII). Below that is shown the response of the manipulated variables □ and□ of the negative phase sequence system control (section III) whichresult from the steady-state d/q controlled variables (in which theintegral-action components also operate) by rotation with the 50 Hz lineangle φ. The 50 Hz oscillation shown therefore results from the directcomponent signals of the integral-action components. The instantaneousamplitude of the oscillation corresponds to the instantaneous Integralvalue. In the next section down, phase currents i_(R) and i_(S) betweenconverter 10 and line filter 12 are plotted (section IV), and in a finalsection the measured filter voltages u_(F,RS) and u_(F,ST) are shown(section V).

The I-components of the controllers 40, 42 for the negative phasesequence system current ensure displacement of the no longer clampedpotential at the separated phase—as before the phase S. By the conductorof the phase S no longer being connected to the line 16 (no longerclamped to the line voltage) at the disconnecting point 14, the voltagesu_(F,RS) and u_(F,ST) can change. Although no shutdown due to fault hasbeen performed in the example, the line fault is easily detectable, e.g.on the basis of the output amplitude of the negative phase sequencesystem control or also on the basis of the line-synchronous fluctuationof the saturation degree. The filter voltage amplitudes likewise changesignificantly, but require the measurement of an external voltage. Inthe case of the situation shown in FIG. 5 of a test on a 400 V linesupply, it follows that the oscillation amplitude is 564 V, the smaller(at the time of consideration) amplitude of the voltages u_(F,RS) andu_(F,ST) decreases with the negative phase sequence system reaction toapprox. 400 V, i.e. by about 30%. This is well outside the line voltagetolerance of −15%.

Marked by way of example of this in the diagrams in FIG. 5 areindividual limit values 70, 72, 74 that can be monitored individually orcumulatively by the monitoring unit 64, namely a limit value 70 for theoscillation amplitude of the degree of saturation at twice linefrequency (section II), a limit value 72 for the level of one of the twointegrators (corresponding to the amplitude of the negative phasesequence system oscillation; section III), and a limit value 74 for aminimum value for the amplitude of one/each phase-to-phase line voltage(section V).

The reaching or undershooting or exceeding of such a limit value 70, 72,74 is a response that is typical of the mentioned coupling of theintegrators 50, 52; 54, 56. By monitoring the respective measured orcalculated values and identifying the limit value violation as aresponse typical of the coupling of the integrators 50, 52; 54, 56, themonitoring unit 64 has automatically detected the underlying line fault.The monitoring unit 64 generates the fault signal 66 to indicate theline fault detected.

Although the invention has been illustrated and described in detail onthe basis of the exemplary embodiment, the invention is not limited bythe example(s) disclosed and other variations will be apparent topersons skilled in the art without departing from the scope ofprotection sought for the invention.

Individual salient aspects of the description presented here may bebriefly summarized as follows: To detect phase failures, in particularphase failures in the form of line faults, an active detection method isproposed which selectively brings about state changes in the systemaffected by the phase/line fault in order to obtain significant anddetectable signal changes and, by means of threshold value comparisons,to detect the disturbed state even under no-load conditions. The activemethod is designed such that, in the undisturbed operating state, thebehavior of the system remains virtually unchanged. Specifically, nodedicated test signals that would represent a deviation from the idealresponse of the output voltage and therefore create undesirable linereactions are injected into the line 16. Instead, the control of theconverter 10 is implemented such that in the event of a phasefailure/line fault (phase interruption), a change in the controlstructure results. This means that functional units (integrators of thecurrent controllers 26, 28; 40, 42) that are not coupled to one anotherduring normal operation are coupled, and feedback loops are createdwhich only then result in a continuously increasing and automaticallydetectable fault signal if the structure change occurs because of thephase/line fault.

What is claimed is: 1.-9. (canceled)
 10. A method for detecting phasefailures in a converter, the method comprising: controlling current ofthe converter with first current controllers of a positive phasesequence system having first integrators and with second currentcontrollers of a negative phase sequence system having secondintegrators, wherein the first integrators and the second integratorsare decoupled without a line disturbance, in the event of a line fault,coupling the previously decoupled first and second integrators; checkingwith a monitoring unit at least one measured or calculated value for aresponse typical of the coupling of the first and second integrators;and with the monitoring unit generating a fault signal when such typicalresponse is detected, wherein the at least one measured or calculatedvalue comprises at least one value selected from the group consisting ofan absolute value of the second integrators, an oscillation amplitude ofa line current absolute value at twice a line frequency, an oscillationamplitude of a filter voltage absolute value at twice the linefrequency, an oscillation amplitude of a DC link voltage at twice theline frequency, and an oscillation amplitude of a saturation degree attwice the line frequency.
 11. The method of claim 10, further comprisingmonitoring with the monitoring unit whether absolute values of thesecond integrators exceed a predefined limit value, and generating withthe monitoring unit the fault signal when the predefined limit value isexceeded.
 12. The method of claim 10, further comprising preventing withlimiting blocks assigned to the second integrators an unlimited ramp-upof the second integrators.
 13. The method of claim 10, wherein thenegative phase sequence system is activated depending on an averagepower consumption of the converter.
 14. The method of claim 10, whereinthe negative phase sequence system is deactivated when an average powerconsumption of the converter exceeds a predefined limit value.
 15. Themethod of claim 10, wherein the second current controllers process, asinput values, a current control deviation from the positive phasesequence system that is rotated by vector rotation into a coordinatesystem of the negative phase sequence system.
 16. A device for detectingphase failures in a converter, comprising: first current controllers ofa positive phase sequence system having first integrators, and secondcurrent controllers of a negative phase sequence system having secondintegrators, the first and second current controllers configured tocontrol a current of the converter, wherein the first integrators andthe second integrators can be operated decoupled without linedisturbance, wherein, in the event of a line fault, the previouslydecoupled first and second integrators are coupled, and a monitoringunit configured to detect at least one measured or calculated value fora response typical of the coupling of the first and second integrators,and to generate a fault signal when such a typical response is detected,wherein the at least one measured or calculated value comprises at leastone value selected from the group consisting of an absolute value of thesecond integrators, an oscillation amplitude of a line current absolutevalue at twice a line frequency, an oscillation amplitude of a filtervoltage absolute value at twice the line frequency, an oscillationamplitude of a DC link voltage at twice the line frequency, and anoscillation amplitude of a saturation degree at twice the line frequency17. A computer program having program code instructions stored on anon-transitory storage medium, wherein when the program codeinstructions are loaded into memory of a processor of the convertercontrol unit and executed by the processor, cause the converter controlunit to detect phase failures in a converter by: controlling current ofthe converter with first current controllers of a positive phasesequence system having first integrators and with second currentcontrollers of a negative phase sequence system having secondintegrators, wherein the first integrators and the second integratorsare decoupled without a line disturbance, In the event of a line fault,coupling the previously decoupled first and second integrators; checkingwith a monitoring unit at least one measured or calculated value for aresponse typical of the coupling of the first and second integrators;and with the monitoring unit generating a fault signal when such typicalresponse is detected, wherein the at least one measured or calculatedvalue comprises at least one value selected from the group consisting ofan absolute value of the second integrators, an oscillation amplitude ofa line current absolute value at twice a line frequency, an oscillationamplitude of a filter voltage absolute value at twice the linefrequency, an oscillation amplitude of a DC link voltage at twice theline frequency, and an oscillation amplitude of a saturation degree attwice the line frequency.
 18. The device of claim 16, comprising aconverter control unit having a processor and a memory storing programcode instructions, wherein when the processor executes the program codeinstructions from the memory, the converter control unit causes thedevice to detect the phase failures in the converter.